CARDIAC CALCIUM CHANNELS EXPRESSED IN XENOPUS OOCYTES ARE MODULATED BY DEPHOSPHORYLATION BUT NOT BY CAMP-DEPENDENT PHOSPHORYLATION

Enhancement of cardiac L-type Ca2+ channel activity by norepinephrine via phosphorylation by protein kinase A (PKA) underlies the positive i notropic effect of this transmitter and is a classical example of an i on channel modulation. However, it is not clear whether the channel pr otein itself (and which subunit) is a substrate for PKA. We have expre ssed various combinations of the cardiac Ca2+ channel subunits in Xeno pus oocytes by injecting subunit mRNAs. Expression of beta or alpha(2) /delta+beta subunits potentiated the native (endogenous) Ca2+ channel currents in the oocyte (similar to T or N but not L-type). This potent iated endogenous current was enhanced by intracellular injection of cA MP or of the catalytic subunit of PKA, and this effect was reversed by the injection of a PKA inhibitor suggesting the presence of basal pho sphatase activity. When a cardiac channel of alpha(1)+beta, alpha(1)+a lpha(2)/delta or alpha(1)+alpha(2)/delta+beta composition was expresse d at levels high enough that the contribution of the endogenous curren t became negligible, cAMP and PKA failed to increase the Ca2+ channel current, whereas PKA inhibitors and the catalytic subunit of protein p hosphatase 1 reduced the amplitude of the current. Reduction of the cu rrent by PKA inhibitors was observed regardless of the presence of the beta subunit, suggesting a major role for the cll subunit in this pro cess. These results suggest that, like in the heart, when expressed in Xenopus oocytes, the cardiac L-type Ca2+ channels are phosphorylated in basal state and dephosphorylation reduces their activity. However, unlike the situation in the heart, the activity of the channel cannot be enhanced by PKA-catalyzed phosphorylation, suggesting that the chan nel is already fully phosphorylated in its basal state.